This invention relates to the field of torque generation and, more particularly, to torque generators for use in motor vehicles. Such torque generators are used, for example, as drivers for brake-by-wire systems and power steering systems. These torque generators comprise an electric motor and a power supply, typically a shared 12-volt battery.
Many compromises must be made in the course of designing a new torque generator. They arise out of conflicting cost, performance and geometrical requirements, and may involve consideration of a myriad of miscellaneous matters, such as manufacturing cost, reliability, maintainability, safety, environmental impact, etc. Even conflicting intellectual property rights may be thrown into the mix. Additionally, the designers of a new torque generator for use in a motor vehicle must consider matters peculiar to the automotive industry, such as customer preferences, industry standards and governmental regulations.
Thus the design of a state-of-the-art torque generator is a daunting task, particularly in the case of torque generators for automotive application. This is due, at least in part, to pervasive, widespread use of 12-volt DC electrical systems in motor vehicles. At least within the United States, 12 V systems have become standard. This has led to wide availability of 12 V electrical components at reasonable cost. It also creates a strong incentive to use 12 V components in torque generators for automotive application. Unfortunately, traditional 12 V motor designs are not able to meet conflicting performance and packaging requirements established by the automotive industry.
Motors that meet free speed and maximum torque specifications require high levels of electrical current. Such currents are possible only if the field windings are fabricated from relatively large gauge, low resistance, (e.g. 30 mΩ) wire. The construction of such large gauge field windings consumes a relatively large amount of copper, thereby increasing the cost of the motor. The use of large gauge field windings also boosts the size of the motor package, so that miniaturization of the torque generator becomes more difficult.
It is customary to optimize the design of brushless DC motors so as perform efficiently in their intended applications. The design process involves selection of an input voltage and selection of a variety of parameters such as the back EMF, the winding inductance per phase, operating points for input voltage, and commutation angle for specific load characteristics. The result of the design is a motor having a speed/torque profile which minimizes power consumption during the expected principal use.
The prior art includes small, brushless 12 V DC motors which are able to meet presently applicable torque requirements, except as to free speed There is a need for a small DC torque generator able deliver high free speed and also high torque, both at low current.